The present invention relates to a resin cage to be employed in a ball bearing.
A ball bearing includes an inner ring, an outer ring, a plurality of balls interposed therebetween, and a cage which holds each of the adjacent balls to be separated from each other In recent years, a resin cage formed by injection molding has been widely used as a cage for such a ball bearing. For example, JP-A-7-190069 describes an example of a resin cage for an angular contact ball bearing. As illustrated in FIG. 6, such a resin cage is formed substantially like a crown, and includes an annular base portion 20, and a plurality of claw portions 21 that are formed to be arranged on the circumference of the base portion 20 at constant intervals and to protrude therefrom. A concave-sphere-like pocket 22 for accommodating a ball is formed between each pair of the adjacent claw portions 21.
Meanwhile, for the convenience of accommodating a spherical object in each of the pockets 22, such a resin cage is shaped so that the thickness of a region extending from a central part to an end part of each of the claw portions 21 sharply increases. Such a sharp change in the thickness may deteriorate the flowability of resin at injection molding with the result of causing injection-molding defects, such as sink marks and air bubbles. In addition, the presence of a thickness difference may cause variation in cooling-down time of resin at injection molding with the result of generation of internal stress and occurrence of shape distortion.
Further, grease for lubrication is enclosed in a space (hereunder referred to as an “internal space”) surrounded by the outer ring and the inner ring of the ball bearing in which the resin cage is mounted. For lengthening the life of the ball bearing, it is desirable to enclose a larger amount of grease in the internal space. However, in a case where grease leaks out of the internal space, problems of safety and appearance occur. Thus, a large internal space is desirable for enclosing a larger amount of grease in the internal space while suppressing grease from leaking therefrom. However, it becomes problematic that the substantial volume of the internal space is reduced by inserting the resin cage into the ball bearing.